1. Field of the Invention
The present invention relates to a moving blade for use in a steam turbine, gas turbine or the like.
2. Description of Prior Arts
It is possible to save energy resources and reduce emission of carbon dioxide and so forth by suppressing a blade profile loss and a secondary flow loss generated in a turbine blade thereby increasing the internal efficiency in a turbine stage.
The well-known prior art to reduce such a turbine moving blade loss has been disclosed, for example, in JP A No. 61-232302, JP A No. 05-187202 and JP A No. 2000-345801.
The above-mentioned JP A No. 61-232302 discloses a blade profile related to a turbine cascade for use in a jet engine or a gas turbine engine and having so-called xe2x80x9cFORE-LOADEDxe2x80x9d type velocity distribution (pressure distribution) wherein the blade profile is made in blunt nose type to increase a load per one blade and as means for reducing aerodynamic losses, the blade suction side maximum velocity point of the velocity distribution and the blade pressure side minimum velocity point are positioned at predetermined locations within a range where SX/SO is from 0.2 to 0.3, which is the respective ratios of the blade surface distance SX from a stagnation point of the leading edge along respective surfaces of the blade suction and blade pressure sides to the entire blade surface length SO from a stagnation point of the leading edge to the trailing edge along respective surfaces of the blade suction and blade pressure sides.
Further, the above-mentioned JP A No. 05-187202 discloses a blade of the turbo-machinery subject to a subsonic state, wherein the blade profile is formed such that a trailing edge portion on the blade suction side is made concave so as to suppress the momentum loss thickness in the trailing edge portion and decrease a blade profile loss.
Furthermore, the above-mentioned JP A No. 2000-345801 discloses a turbine moving blade which is formed such that it is FORE-LOADED or mid-loaded type on the inner-ring side and is AFT-LOADED type on the outer-ring side.
Generally, turbine stages can be classified into impulse stages and reaction stages according to the blade root reaction degree. Here, the ratio of a heat drop (variation of enthalpy) in a moving blade to a total heat drop in a turbine stage is called the degree of reaction, and the blade root reaction degree of the former stage is 0% and that of the latter stage is 50%. However, it is difficult to strictly classify actual turbine stages in such a manner. Accordingly, thereafter in the following description, a turbine stage with a blade root reaction degree of from 5 to 10% is called an impulse stage, a turbine stage with a blade root reaction degree of nearly 50% is called a reaction stage and a turbine stage with an intermediate blade root reaction degree of over 10 and less than 50% is called low-reaction type or a low-reaction stage (thereafter, referred to as xe2x80x9clow-reaction stagexe2x80x9d).
As the degree of reaction at the root approximates to 0%, a turning angle of the turbine moving blade profile increases. As a result, it is necessary to increase the curvature of the blade suction side. Further, in the blade profile in which a maximum blade loading location is located on the leading edge side (FORE-LOADED), a decelerated flow (adverse pressure gradient) area extending from the blade suction side maximum velocity point (minimum pressure point) of the velocity distribution toward the trailing edge becomes relatively long, which makes the boundary layer grow, increasing the thickness of the boundary layer at the end of the trailing edge, thus causing a friction loss. If such a cascade is specifically applied to an impulse stage or a low-reaction stage of a moving blade, a flow cannot catch up with the large curvature of the blade suction side, which may result in the flow being separated and causing an excessive loss.
Furthermore, since a secondary flow results from a pressure difference between the blade suction side and the blade pressure side, the further out on the leading edge side a maximum blade loading position is, the earlier a secondary flow vortex starts to grow, which tends to increase the secondary flow loss.
The shape of a blade has a close relationship with the stage reaction degree. Specifically, the degree of reaction has a great effect on the blade turning angle (camber). The optimum blade loading distribution shape that minimizes a blade profile loss is considered to exist in accordance with the blade turning angle. It is generally known that a blade having a maximum blade load on its trailing edge side (AFT-LOADED) rather than on its leading edge side has less blade profile loss because the accelerated flow area on the suction-side blade surface is enlarged thereby suppressing the growth of the boundary layer. Furthermore, from the aspect of the secondary flow suppression, employing a blade having a maximum blade load on its trailing edge side reduces the secondary flow loss, which makes it possible to lower both blade profile and secondary flow losses.
When designing such a blade profile, however, a blade surface pressure distribution which minimizes a loss, the shape of a blade loading distribution, and the division rate of the blade load are unknown. The above-mentioned prior art does not take such points into consideration. Specifically, the blade profile turning angle of a moving blade root portion in an impulse stage and a low-reaction stage for a steam turbine is set to be approximately 110 degrees or more. This is larger than approximately 80 degrees, the turning angle of a moving blade in a reaction stage. As a result, not only the blade profile loss but also the secondary flow loss ends up contributing to a major loss among stage losses.
In view of the foregoing, an object of the present invention is to provide a turbine moving blade capable of reducing aerodynamic losses, such as a blade profile loss and a secondary flow loss.
To achieve the above-mentioned object, the present invention provides a turbine moving blade operated by working fluid, wherein a blade profile is formed such that the pressure distribution defined by the suction-side surface pressure of the moving blade drops in two stages in the area from the leading edge to the minimum pressure point.